This invention relates to a magnifying apparatus especially for use by the visually impaired, and more particularly, to a two lens stand magnifier with variable power and zooming capability.
The prior art is replete with numerous stand magnifiers which include, for instance, a plus lens which enables a person to view objects more clearly. For example, stand magnifiers consisting of a single plus (convex) lens and a holder are sold by COIL, Bausch & Lomb, Visolette, and others. Examples of such magnifiers include those shown in British Registered Design Nos. 1,037,361, 1,037,062, and 1,049,389, U.S. Design Pat. Nos. 303,977 and 306,175, corresponding to the first and second of those British registered designs, respectively, U.S. Pat. No. 4,944,574, also corresponding to the first of those registered designs, and U.S. Pat. Nos. 2,199,107 and 4,190,322. Although not illustrated in any of these patents, the COIL stand magnifier Nos. 5428 and 5123 are probably the most common medium power units in the industry.
The position of the lens of most such stand magnifiers is normally fixed with respect to the object to be magnified at a distance slightly within the focal length of the plus lens. Thus, a magnified virtual image is produced behind the object as shown schematically in FIG. 1. Perhaps the biggest disadvantage of such magnifiers is that the lens must be very close to the object to be magnified. Thus, for medium and high powered stand magnifiers, there is insufficient clearance, referred to as the "working distance", to allow the user of the magnifier to write or draw conveniently on paper positioned under the lens; further, the lens blocks illumination of the object.
The prior art systems are also generally of one power; thus magnifiers of different powers are required for different tasks. An example is the COIL 4210, 4212, 4215, and 4220 stand magnifiers having nominal 10.times., 12.times., 15.times., and 20.times. magnifications, respectively. Previous magnifiers which claimed to be of variable power are really only variable focus as they are merely two plus lenses. As example is provided by the apparatus shown in U.S. Pat. No. 4,859,032. In these variable focus magnifiers, when one lens is moved slightly with respect to the other, variable focus results to neutralize refractive errors of the observer only. The magnification power does not change.
In referring to "magnification power", it is useful to note that stand magnifiers are specified by the equivalent power of the magnifier and eye system (F.sub.e), by the eye to image distance (ID), by the transverse magnification of the image to object ratio (M.sub.T), and/or by the magnification of the system (F.sub.e /4). A useful equation (taken from I. Bailey, The use of fixed-focus stand magnifiers", 72 Optom. Monthly 37-39 (1981)) for expressing F.sub.e is ##EQU1## For example, the above-mentioned COIL 5428 stand magnifier has an F.sub.e of 8.25 diopters at an eye to image distance ID of +2.50 diopters, or 40 cm, and the COIL 5123 has an F.sub.e of 17.0 at this image distance. Thus, magnification for these magnifiers is F.sub.e /4, or 2.times. for the 5428 and 4.25.times. for the 5123.
A well known lens system used for zoom magnification purposes is a camera zoom lens system, shown schematically in FIG. 2. The structure of a camera zoom lens system, or variable power lens system, is not adaptable for use as a stand magnifier because a camera zoom lens is designed to perform a different function. Camera zoom lens systems are generally noncompact, multi-element lens systems designed to take a far away object (shown in FIG. 2 as the parallel rays entering minus lens L.sub.1) and, with a many-fold change in magnification, project a real image I of that object which is continually sharp and clear on the film plane. The simplest two lens zoom system for a camera such as is shown in FIG. 2 requires that the plus lens L.sub.2 move with respect to the minus lens L.sub.1. To function appropriately, the object must be positioned beyond the focal point (also not shown) of the system. If an object is positioned within the focal point of the camera zoom lens system, as it is in the case of a stand magnifier, a real image will not be produced on the film plane. For any stand magnifier, including the stand magnifier of the present invention, the image from the object to be magnified is a virtual image at about 40 cm (+2.50 diopters) from the observer's eye.
Also, camera zoom lenses are characterized by certain disadvantages and limitations which make their structure unsuitable for use as a magnifier. For instance, multiple lens are required in zoom lens systems to accomplish the desirable goals of minimizing image aberrations and eliminating image movement. Of course, multiple lenses cannot economically be incorporated into and are not necessary in a stand magnifier, and, even if they could be, they would add to the weight and size of the magnifier.
Another well known apparatus used for magnification is the microscope. The microscope includes two highly corrected compound positive lenses used to obtain a magnification much greater than that obtainable with a simple stand magnifier. Microscopes generally have magnification powers of 25.times. and higher, whereas simple stand magnifiers have magnification powers of approximately 1.5.times. to 20.times.. An object must be placed beyond the focal length of the first lens of the microscope such that a real image is produced at or inside the focal length of the second lens. Great precision is required when building the microscope components and multiple powers are usually only obtained by switching between objective lenses of different powers. Thus, they are typically expensive, have low field of view, are highly corrected, and of high power.
Therefore, it is an object of the present invention to provide an improved and affordable two lens stand magnifier providing variable magnification zoom powers.
It is another object of the present invention to provide a stand magnifier with optics entirely different from that of present stand magnifiers of only a single plus lens, different from camera zooms that form a real image with a movable plus lens, and different from microscope optics with two plus lens systems.
It is another object of the present invention to provide an improved stand magnifier having an increased working distance between the lens and the object or print being magnified.
It is another object of the present invention to provide a stand magnifier in which the virtual image remains in focus at a convenient viewing distance, for instance, 40 cm or +2.50 D, for all powers.
It is yet another object of the invention to provide a method for varying the magnification power and the lens powers and spacings of a magnifier while maintaining the image in focus.
Still another object of the present invention is to provide a "family" of variable power zoom magnifiers, e.g., low, medium, and high power stand magnifiers, each magnifier in the family having approximately a two-fold increase in power over its range or magnification.